Optically variable devices are used in a wide variety of applications, both decorative and utilitarian, for example such devices are used as security devices on commercial products. Optically variable devices can be made in numerous ways to achieve a variety of effects. Examples of optically variable devices include the holograms imprinted on credit cards and authentic software documentation, colour-shifting images printed on banknotes, and enhancing the surface appearance of items such as motorcycle helmets and wheel covers. Security devices bearing printed images are applied to currency, travel documents, drivers' licenses, lottery tickets, and objects such as bottles containing pharmaceuticals or other products where authenticity and or security of the product or brand is very important.
Optically variable devices can be made as film or foil that is pressed, stamped, glued, or otherwise attached to an object, and can also be made using optically variable pigments. One type of optically variable pigment is commonly called a colour-shifting pigment because the apparent colour of images appropriately printed with such pigments changes as the angle of view and/or illumination is tilted. A common example is the “20” printed with colour-shifting pigment in the lower right-hand corner of a U.S. twenty-dollar bill, which serves as an anti-counterfeiting device.
Some anti-counterfeiting devices are covert, while others are intended to be noticed. Unfortunately, some optically variable devices that are intended to be noticed are not widely known because the optically variable aspect of the device is not sufficiently dramatic. For example, the colour shift of an image printed with colour-shifting pigment might not be noticed under uniform fluorescent ceiling lights, but more noticeable in direct sunlight or under single-point illumination. This can make it easier for a counterfeiter to pass counterfeit notes without the optically variable feature because the recipient might not be aware of the optically variable feature, or because the counterfeit note might look substantially similar to the authentic note under certain conditions.
Optically variable devices can also be made with magnetically alignable pigments that are aligned with a magnetic field after applying the pigment (typically in a carrier such as an ink vehicle or a paint vehicle) to a surface. However, painting with magnetic pigments has been used mostly for decorative purposes. For example, use of magnetic pigments has been described to produce painted cover wheels having a decorative feature that appears as a three-dimensional shape. A pattern was formed on the painted product by applying a magnetic field to the product while the paint medium still was in a liquid state. The paint medium had dispersed magnetic non-spherical particles that aligned along the magnetic field lines. The field had two regions. The first region contained lines of a magnetic force that were oriented parallel to the surface and arranged in a shape of a desired pattern. The second region contained lines that were non-parallel to the surface of the painted product and arranged around the pattern. To form the pattern, permanent magnets or electromagnets with the shape corresponding to the shape of desired pattern were located underneath the painted product to orient in the magnetic field non-spherical magnetic particles dispersed in the paint while the paint was still wet. When the paint dried, the pattern was visible on the surface of the painted product as the light rays incident on the paint layer were influenced differently by the oriented magnetic particles.
Similarly, a process for producing a pattern of flaked magnetic particles in fluoropolymer matrix has been described. After coating a product with a composition in liquid form, a magnet with a magnetic field having a desirable shape was placed on the underside of the substrate. Magnetically orientable flakes dispersed in a liquid organic medium orient themselves parallel to the magnetic field lines, tilting from the original planar orientation. This tilt varied from perpendicular to the surface of a substrate to the original orientation, which included flakes essentially parallel to the surface of the product. The planar oriented flakes reflected incident light back to the viewer, while the reoriented flakes did not, providing the appearance of a three dimensional pattern in the coating.
Special effect optically variable coatings may be in the form of flakes in a carrier or a foil and may be color shifting, color switching, diffractive, reflective, any combination of color shifting or color switching and diffractive, or may have some other desired feature. Field-alignable flakes or particles may include magnetic metallic, multi-layer metallic, magnetic flakes having an optical interference structure, magnetic effect pigments, magnetic optically variable, magnetic diffractive, and magnetic diffractive optically variable.
Printing with special effect inks can be done using a silk screen or can be done by any conventional means of applying ink to a substrate. In a preferred embodiment of this invention an Intaglio ink process is used to apply the ink. Non-limiting examples include gravure, flexographic, and offset methods.
Although special effect coatings forming images are well known, this invention provides a novel an inventive structure that conveniently limits the perceived travel of a dynamic effect in an image thereby differentiating two regions printed with the same ink. Unexpectedly, while limiting the perceived dynamic effect, the optically variable effects are not limited to a single region.
It is an object of this invention to provide a printed security device that forms a image printed with the same ink, whereby two lined or pixilated regions having different width lines have different perceived optical effects based in differences in the cross sectional surface of the printed lines.
The inventors of this application have discovered that when plural parallel spaced lines printed in color shifting ink are very narrow or pixels are very small, that color shifting effects can be seen. The inventors have also discovered that when flakes within the ink forming these lines or pixels are magnetically aligned, the effects provided by the magnetic alignment by and large are not visible. Notwithstanding, the inventors have also discovered that if the line width or pixels size is increased sufficiently, both color shifting effects and effects associated with magnetic alignment is perceptible without magnification. This is also a convenient way in which to limit the perceived travel of a dynamic effect while using the same ink but varying thickness and height. Thus, it is the overall surface area of the ink across a printed line that determines whether features associated with its magnetic alignment can be perceived.